1. Field of the Invention
This invention is drawn to reaction injection molding (RIM) processed polyurea compositions having a modulus greater than 100,000 psi, heat sag less than one inch (at 325.degree. F./1 hour) and good damage resistance, blends employed in the production of such compositions and a process for making such polyurea compositions.
2. Description of the Prior Art
Reaction injection molding is a rapid polymerization process commonly employed to produce polyurethanes and polyureas having elastomeric to rigid properties. This technology is employed in the production of automotive parts, such as fascias and bumpers. In the RIM process, two highly reactive streams of chemicals are brought together under high pressure, approximately 2,000 psi, in a small mixing chamber where the streams are impingement mixed by being sprayed directly into each other. The mixed material flows immediately into a mold where the chemical reaction continues and the part is cured. One of the streams, commonly referred to as Component A, contains a polyisocyanate. The other stream, commonly referred to as Component B, contains a high molecular weight isocyanate reactive polymer and a chain extender. The RIM process is widely known and a detailed description of the technology may be found in numerous texts. See in particular "Reaction Injection Molding" edited by Walter E. Becker, VanNostrand Rienhold Publishers, 1979.
High modulus RIM polymers are likely candidates to replace steel body panels, provided they are capable of withstanding on-line elevated temperature finishing operations. Such capacities are indicated by the heat sag of the polymer.
The polyurethanes of the prior art characteristically have high modulus and poor thermal resistance, and, thus are unsuitable for use in such capacities. On the other hand, while the prior art polyureas have high thermal resistance, attempts to isolate high modulus polyurea moldings have been unsuccessful. Such failures are chiefly due to two factors:
1. Rapid gelation. This phenomena is due to the virtually instantaneous reaction of amine-terminated polyethers and isocyanates. While aromatic amine chain extenders are often employed to retard the gel time, gelation nevertheless usually occurs prior to the mold being completely filled. As a result, the moldings obtained are not useable.
2. The brittleness of the resulting polymers. The prior art recognizes that higher modulus RIM polyureas and polyurethanes are obtained when the amount of the reaction product of isocyanate plus chain extender (hardblock) in the final product is increased. Unfortunately, polyureas having greater than 50 weight percent hardblock are extremely brittle, prior to post-curing, and therefore usually can not be demolded without cracking.